Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits.
A digital micromirror device (DMD.TM.), sometimes referred to as deformable micromirror device, is a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. While some micromechanical devices, such as pressure sensors, flow sensors, and DMDs have found commercial success, other types have not yet been commercially viable.
Digital micromirror devices are primarily used in optical display systems. In display systems, the DMD is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, DMDs typically operate in a digital bistable mode of operation and as such are the core of the first true digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane which, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schlieren optics were used to illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. Schlieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast ratios, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer tended to deform over a large region, however, which scattered light impinging on the deformed portion. Torsion beam devices use a thin metal layer to form a torsion beam, which is referred to as a hinge, and a thicker metal layer to form a rigid member, or beam, typically having a mirror-like surface: concentrating the deformation on a relatively small portion of the DMD surface. The rigid mirror remains flat while the hinges deform, minimizing the amount of light scattered by the device and improving the contrast ratio of the device.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by fabricating the mirror on a pedestal above the torsion beams. The elevated mirror covers the torsion beams, torsion beam supports, and a rigid yoke connecting the torsion beams and mirror support, further improving the contrast ratio of images produced by the device.
Consumer display applications have also been evolving as consumers have come to expect increasing image resolution and quality. For example, business projectors will soon be expected to have .mu.XGA image resolution, that is be able to produce images resolutions of 1024 .times.768 pixels. Micromirror-based display systems are difficult to scale to higher resolutions since more elements must be added to the micromirror array. For example, increasing the number of elements in the array increases the size of the device--thereby lowering the number of devices fabricated on a single semiconductor wafer and increasing the size of the projection optics.
While enlarging the size of the array entails several disadvantages, it is also difficult to reduce the size of the modulator elements in order to add elements to the array. The micromirror elements are micromechanical machines that cannot easily be scaled. Furthermore, the semiconductor substrate underneath the micromirrors is filled by a memory array that holds data for each micromirror. The existing memory cells cannot be reduced enough to fit under significantly reduced micromirrors without violating the minimum design rules governing the silicon processing of the SRAM cell. What is needed is a new memory design to support the fabrication of smaller micromirrors that provide less space to fabricate the memory cell.